1987 - 2017: Thirtieth Anniversary!
Best viewed in "landscape" mode.
I have a lot more to write here, but to keep it simple, "real" speakers don't need the sort of "subwoofers" that have been popularized by consumer home theater systems commonly available.
A real speaker already makes better bass than they do. To augment a good speaker with a subwoofer it should basically cover the range from 16 to 64 Hz and handle about 500 watts.
I do, however, amuse myself sometimes with sketches of cabinets with four or eight of my 8" woofers, and still look at sources for 500-800 watt amplifers.